A known RF power amplifier is illustrated in FIG. 1. The corresponding equivalent circuit diagram is illustrated in FIG. 2. The known amplifier 1 comprises a package having an output lead 2, an input lead 3, and a flange 4. An active die 5, on which an RF power transistor 6 is arranged, is mounted on flange 4. An input of RF power transistor 6, such as a gate, is connected to a bondpad bar 7, and an output of RF power transistor 6, such as a drain, to a bondpad bar 8. Packaged RF power amplifier 1 further comprises a first passive die 9, a second passive die 10, and a third passive die 11. On each die, an integrated capacitor C1, C2, C4 is arranged that has two terminals of which one is grounded. Within the context of the present invention, a grounded terminal refers to a terminal that is electrically connected to flange 4. The other terminal is connected to a bondpad bar assembly comprising bondpad bars 12, 13, 14. Each of the dies 9, 10, 11 as well as the connecting bondwires are arranged inside the package.
Within the context of the present invention, an active die is a semiconductor die on which the RF power transistor is arranged, and a passive die is a die, preferably but not necessarily made from semiconductor material, on which one or more passive components are realized.
A first inductor L1 connects the output of RF power transistor 6 to output lead 2. This inductor comprises a plurality of first bondwires 19 that extend in a first direction between bondpad bar 8 and output lead 2. A second inductor L2 connects the output of RF power transistor 6 to the non-grounded terminal of first capacitor C1. This inductor comprises a plurality of second bondwires 17 that extend between bondpad bar 8 and bondpad bar 13. A third inductor L3 connects the first terminal of first capacitor C1 to the non-grounded terminal of the second capacitor C2. This inductor comprises one or more third bondwires 18 that extend between bondpad 13_1, which is electrically connected to bondpad bar 13, and bondpad bar 14.
A fifth inductor L5 connects input lead 3 to the non-grounded terminal of fourth capacitor C4. This inductor comprises a plurality of fifth bondwires 15 that extend between input lead 3 and bondpad bar 12, which bondpad bar is electrically connected to the non-grounded terminal of fourth capacitor C4. A sixth inductor L6 connects bondpad bar 12 to the input of RF power transistor 6. This inductor comprises a plurality of sixth bondwires 16. Inductors L5, L6 and capacitor C4 constitute an input impedance matching network.
As illustrated in FIG. 1, second capacitor C2 is located on passive die 11 next to active die 5, whereas the first and fourth capacitors C1, C4 are arranged in between active die 5 and output lead 2 or input lead 3, respectively.
Now referring to FIG. 2, a parasitic output capacitance is present at the output of RF power transistor 6. This capacitance, modeled by Cds, deteriorates the performance of RF power transistor 6 at the operational frequency, which typically lies in a range between 1 and 3 GHz, although other frequency ranges are not excluded.
FIG. 2 illustrates the known solution to overcome this problem. The output network formed by L2, L3, C1, and C2 is configured to resonate with Cds at or close to the operational frequency. More in particular, at or close to the operational frequency, the output network will act as a shunt inductor. This inductor will display a parallel resonance with Cds such that the impact of the latter on the RF performance at the operational frequency is mitigated. Typically, the shunt inductor is largely determined by L2.
C2 is much larger than C1. C2 will, at a relatively low frequency, display a parallel resonance with the inductance associated with the biasing network. This inductance is modelled by Lfeed in FIG. 2. It should be noted that the invention is not limited to the particular position at which the biasing currents are introduced in the circuit.
The parallel resonance of C2 and Lfeed will introduce a first peak in the effective impedance seen at the drain of the transistor. Another resonance occurs at a higher frequency substantially corresponding to the resonance frequency of C1 and L3. By properly choosing the component values for L2, L3, C1, and C3 a desired impedance behavior can be realized in the frequency range typically associated with second order intermodulation products. In this range, the impedance seen by RF power transistor 6 should be as low as possible to avoid performance degradation.
An important design parameter for packaged RF power amplifiers is the power that can be generated within a given package size. A higher power density, expressed in Watts per unit package area, enables a more compact design. A further important parameter is the efficiency with which the power is generated, such as the power added efficiency. A high efficiency indicates that little power is dissipated inside the package. This has positive consequences for the amount of cooling that is required for cooling the packaged RF power amplifier and for the overall power budget of the system.
A packaged radiofrequency (RF) power amplifier according to the preamble of claim 1 is known from EP2388815A1[DRG1]. This document describes an alternative implementation of the circuit in FIG. 1. Here, capacitor C2 is integrated on the active semiconductor die and capacitor C1 is integrated on a passive semiconductor die that is arranged inside the package in between the active die and the output lead. A further packaged radiofrequency (RF) power amplifier is known from US2007024358A1[DRG2].